An electronegative inductive discharge instability

An instability has been observed with Langmuir probes in cylindrical inductive discharges operated with low electronegative gas pressures. The instability consists of ion density modulations that propagate axially and radially at the ion acoustic speed. Measurements and calculations of negative ion concentrations in oxygen discharges suggest that the radial distribution and the magnitude of the density drops are consistent with the negative ion population. The instability appears to be a periodic expulsion of most negative ions from the plasma core. The cause of the modulations remains to be clarified. The instability may provide valuable information on negative ion concentrations and temperatures in a low pressure regime where measurements of those parameters are difficult.